Phase-separation member

ABSTRACT

A phase-separation member comprises a porous substrate ( 11 ) containing void spaces, a microporous polymer material ( 14 ) which at least partially impregnates the porous substrate by entering into the void spaces, and a layer of a fluoropolymer applied to the outer face of the coagulated polymer material so that the layer ( 16 ) of fluoropolymer material remains predominantly at the surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to improvements in phase-separation members, inparticular solid-liquid industrial separation, filter media such asfilter cloths or filter belts, for all recongnised pressure and vacuumfiltrations systems, such as rotary drum filters, belt filter presses,etc. The term phase-separation members also includes papermaking fabricssuch as forming fabrics, press felts, dryer fabrics or transfer fabrics.The invention also relates to corrugator belts and conveyor belts. Thesemembers are preferably water permeable.

2. Prior Art

It is known for such members particularly but not exclusively in thecase of filters and papermaking fabrics, to provide a substrate, forexample of a woven fabric, a spiral link fabric, a sintered sheet, aneedlefelt or nonwoven textile, or a porous film. Such members are oftenmade from or include fibres or particles of a low surface energymaterial such as a polyolefin, typically polypropylene.

To improve properties of the filter or papermaking fabric, such asfilter cake release, it is desirable to be able to coat the substratewith a fluoropolymer such as PTFE. However, low surface energy materialssuch as polyolefins bond very poorly to fluoropolymers, and as a resultdurable release coatings are difficult to achieve on substrates of thekind mentioned.

We have previously proposed a process for filtering kaolin particles, inGB-A-2316015, using a filter comprising a fabric substrate coated orimpregnated with a coagulated polymer. We have also proposed, in GB-A-2,288, 755 a coated filter fabric comprising a cloth impregnated with acoagulated polymer latex which is coagulated in situ after impregnationin steam. These disclosures do not consider any problem other thanprovision of a microporous polymer medium within the voids of a fabricsubstrate, and do not suggest any solution to the problem of bonding afluoropolymer coating to a low surface energy material substrate. Alsoin WO98/07925 we disclose a filter etc fabric which may be renderedporous by preinclusion of hollow yarns or fibres which may be opened byabrasion, to provide passages in the fabric. It is suggested that thefabric may be encapsulated in a polymer although it is not suggestedthat a coagulatable or microporous material be used, and separately thatfluoropolymer may be used to improve non-stick and contaminant resistantproperties. It is not suggested that there is any problem involved withthe use of fluoropolymer coatings, or that the use of a microporouslayer might mitigate this.

An object of the invention is to provide an improved phase-separationmember or the like in which a durable bond is achievable between alow-surface energy substrate material, and a fluoropolymer.

SUMMARY OF THE INVENTION

According to the invention a phase-separation member or the likecomprises a porous substrate of a low surface energy material havingvoid spaces therein, and a fluoropolymer layer applied to at least oneouter surface of the member, characterised in that a microporous polymermaterial is provided which at least partially impregnates said poroussubstrate, and in that the fluoropolymer layer is applied to an outersurface of said microporous polymer material such that the fluoropolymerlayer remains predominantly at such outer surface, the microporouspolymer serving to provide a bond between the fluoropolymer layer andthe low surface energy material of the porous substrate.

The microporous polymer may be a coagulated polymer which is coagulatedduring or after impregnation of the substrate thereby.

The microporous layer can be of any synthetic or natural polymer whichcan be dissolved in a solvent, for example a polyurethane, silicone,fluoroelastomer or rubber.

The porous substrate may comprise or include particles, yarns or fibresof a low surface energy material such as a polyolefin, especiallypolypropylene. The porous substrate may be in the form of any of thesubstrates listed hereinbefore, i.e. woven or nonwoven fabric, knittedstructures, needlefelt fabric, porous film, sintered sheet of metal orsynthetic particles or fibres, or spiral link fabric.

All the above substrate structures include void space into which thecoagulated polymer can at least partially penetrate. Preferably thesubstrate structure is impregnated to a substantial extent, e.g. to halfor more than half the thickness of the substrate and provides a coatingnot only over the substrate but within the void space, of the yarns,fires, or particles forming the substrate to provide a filter medium ofmuch finer pore size than would be provided by the substrate unaided.Advantageously, the coagulated polymer impregnates the substrate, i.e.provide a coating on both major surfaces as a well as impregnating thesubstrate.

The fluoropolymer used in coagulation or to coat the substrate afterimpregnation of the latter with the coagulated polymer may comprise asynthetic fluorinated elastomer such as polymers or copolymers ofvinylidene fluoride; pentafluoropropene; tetrafluoroethylene;hexafluoropropene; e.g. vinylidenefluoride-pentafluoropropene-tetrafluoroethylene terpolymer, orvinylidene fluoride-hexafluoropropene-tetrafluoroethylene terpolymer.Fluoropolymers such as tetrafluoroethylene PTFE or other fluoro-alkenepolymers however may be used.

The coagulatable polymer may be a relatively low viscosity material, inthe range 300-1000 cP, e.g. of about 500 cP, and have a relatively highsolids content. The low viscosity enables the polymer to penetratesubstantially into the substrate structure, entering into the voids orinterstices between fibres, yarns or particles making up the substrate.

The coating and impregnating layer of coagulatable polymer may beapplied to the substrate as the polymer is coagulating, for exampleusing DMF in a 5-30% solids solution. The coagulated polymer istypically a low surface energy polymer.

Coagulation may be achieved by heating the impregnated coated textilesubstrate in the presence of a heat coagulant. Suitable heat coagulantsinclude vinyl alkyl ethers and derivatives thereof; polyacetals;polythio ethers; poly (ethylene oxide) and derivatives thereof; and poly(propylene/ethylene oxide) and derivatives thereof. Heating to atemperature of about 70° C. is sufficient to effect coagulation.

An alternative method of coagulation is by adding a suitable electrolyteand/or varying the pH of the polymer latex. For example, with cationicpolymers, coagulation may occur at an alkaline pH and for anionicpolymers coagulation occurs at an acidic pH.

The coagulatable or coagulating polymer may be applied by any coatingtechnique such as knife coating, dip-coating, screen printing orspraying, padding or using reverse roller techniques.

The fluoropolymer coating is in turn preferably applied to the outersurface of the coagulated polymer coated substrate by lick coating,spraying, foaming or paste spreading as a particulate dispersion, withfor example 40-70 wt % solids and particle size 0.1-0.5 microns, ontothe receiving surface and then the liquid component of the dispersion(which is preferably water for environmental reasons) is removed e.g. byevaporation pressing in a mangle, or suction into a slot, to leave awell-bonded low surface energy coating. Consolidation of thefluoropolymer coating can be improved by calendering the coated fabricto consolidate the structure, thereby improving retentivity (i.e.capture of filtrate particles) and smoothness (for better cake release).

The smooth fluoropolymer coating provides the microporous structure andany yarn knuckles or floats proud of said structure with enhancedabrasion resistance, as well as providing the fabric with good cakerelease properties. Filtrate particles are captured in the coagulatedpolymer forming the microporous structure.

Bodies such as hollow glass microbeads may be used to fill voids in thesubstrate, in place of or in addition to the coagulated polymer.

The coagulated polymer may be applied at a weight of 20-200 g/m²,producing a coating substrate (made from e.g. a polymolefin, polyester,polyamide, or PANO, with a weight of 50-2000 g/m², before calendering.

These together with other objects and advantages which will becomesubsequently apparent reside in the details of construction andoperation as more fully hereinafter described and claimed, referencebeing had to the accompanying drawings forming a part hereof, whereinlike numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

A number of possible embodiments of phase-separation or the like membersaccording to the invention will now be described, by way of example,with reference to the accompanying drawings, wherein

FIG. 1 is an enlarged fragmentary cross section of a filter belt inaccordance with the invention, incorporating a woven textile fabricsubstrate;

FIG. 2 is a similar view to FIG. 1 of a papermachine fabricincorporating a spiral link fabric substrate;

FIG. 3 is a similar view of a filter fabric incorporating a substrateformed of a sheet of sintered particles;

FIG. 4 is a similar view of a papermachine press felt, having asubstrate of a nonwoven textile fabric; and

FIG. 5 is a similar view of a member incorporating a porous filmsubstrate, which may be used in any of the above uses, or for example asa conveyor belt.

DESCRIPTION OF THE INVENTION

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the from this detailed description.

In FIG. 1, a belt 10 is shown which may be suitable for use as a filterbelt. This comprises a woven substrate 11, shown diagrammatically ascomprising CD yarns 12 and MD yarns 13 interwoven therewith, in atypical float and knuckle pattern. The substrate 11 is impregnated witha coagulated polymer material 14, as shown by cross-hatching.

The upper side floats 15 of the MD yarns 13 are however exposed abovethe material 14, which thereby presents a wear resistant surface proudof the coagulated polymer layer 14. The layer 14 extends below thefabric 11, and partially encapsulates the fabric on this lower side.

The floats 15, and the layer 14 of coagulated polymer material is coatedwith a layer 16 of a fluoropolymer material. This promotes release ofany material such as filter cake collecting on the surface of the layer16.

FIG. 2 shows a papermachine belt 20 having a spiral link fabricsubstrate 21, which is partially impregnated with a layer 22 of acoagulated polymer material, which is in turn coated on an upper surfacewith a layer 23 of a fluoropolymer material. The layer 22 may bereinforced with a woven or nonwoven layer, or a fibrous batt as commonlyused in the structure of composite papermachine belts.

FIG. 3 shows a filter fabric 30, comprising a substrate 31 comprised ofsintered particles or beads of solid or porous polymer material, that ispartially melted under pressure so that contacting surfaces of theparticles or beads are bonded on resetting of the polymer, leaving voidsand interstices between the particles. Other sintered materials, such asmetals, or thermoplastic fibres may be used for substrate 31.

Substrate 31 is coated and partially impregnated with a layer 32 of acoagulated polymer material, which penetrates into the voids andinterstices of substrate 31 to at least half way through the thicknessof the substrate, as shown by cross-hatching in the drawing. In turn,layer 32 is coated with a coating layer 33 of a fluoropolymer material.

In FIG. 4, a papermachine press felt 40 comprises a substrate 41 formedof a fibrous nonwoven batt or layer. A coagulated polymer layer 42 isprovided on the substrate 41, and penetrates into the interstices of thefabric, between fibres to impregnate the substrate 41 to a substantialextent, as suggested by cross-hatching. The layer 42 may completelyimpregnate the substrate 41.

A fluoropolymer coating 43 is applied to the layer 42.

FIG. 5 shows a further embodiment of material 50 according to theinvention which may be used as suggested in any of the above describedembodiments, or for example in conveyor belting. This material 50comprises a substrate 51 of a porous (e.g. foamed) membrane of plasticsmaterial, which is rendered water permeable by its porous nature. Thesubstrate 51 is coated with a layer 52 of coagulated polymer material,which penetrates into the porous structure of the substrate 51, althoughthis cannot be conveniently illustrated in the drawing. Layer 52 iscoated in its turn with a layer 53 of a fluoropolymer.

In the foregoing embodiments, the coagulated polymers, the fluoropolymercoatings, and the materials of the substrates are selected from theexamples set out earlier in the above description.

What is claimed is:
 1. A durably bonded phase-separation membercomprising a porous substrate of a low surface energy material havingvoid spaces therein, a microporous polymer material at least partiallyimpregnating said porous substrate, and a fluoropolymer layer applied toan outer surface of said microporous polymer material so as to remainpredominantly at said outer surface, said microporous polymer materialserving to provide a durable bond between said fluoropolymer layer andsaid low surface energy material of said porous substrate.
 2. Thephase-separation member according to claim 1, wherein said microporouspolymer material is of a natural polymer which can be dissolved in asolvent.
 3. The phase-separation member according to claim 1, whereinsaid microporous polymer material is any one of a polyurethane, asilicone, a fluoroelastomer, or a rubber.
 4. The phase-separation memberaccording to claim 1, wherein said low surface energy material is apolyolefin.
 5. The phase-separation member according to claim 1, whereinsaid porous substrate comprises any one of a woven or nonwoven fabric, aknitted or a needle felt fibre, a porous film, a sintered sheet ofsynthetic particles, or a spiral link fabric.
 6. The phase-separationmember according to claim 1, wherein said porous substrate isimpregnated to at least half of a thickness of said substrate and saidpolymer material provides a coating over said porous substrate, andwithin said void spaces, to provide a filter medium of finer pore sizethan said porous substrate.
 7. The phase-separation member according toclaim 6, wherein said microporous polymer coagulates and impregnatessaid porous substrate substantially completely.
 8. The phase separationmember according to claim 1, wherein said fluoropolymer layer includes asynthetic fluorinated elastomer.
 9. The phase separation memberaccording to claim 8, wherein said fluoropolymer layer is any one of: apolymer or copolymer of vinylidene fluoride; pentafluoropropene;tetrafluoroethylene; or hexafluoropropene.
 10. The phase separationmember according to claim 9, wherein said polymer or copolymer ofvinylidene fluoride is any one of:vinylidenefluoride-pentafluoropropene-tetrafluoroethylene terpolymer; orvinylidene fluoride-hexafluoropropene-tetrafluoroethylene terpolymer.11. The phase separation member according to claim 1, wherein saidmicroporous polymer material is coagulatable and has a viscosity in arange of 300-1000 cP.
 12. The phase separation member according to claim1, wherein hollow glass microbeads are used to fill the void spaces onsaid substrate.
 13. The phase-separation member according to claim 1,wherein said microporous polymer material is of a synthetic polymerwhich can be dissolved in a solvent.
 14. A durably bondedphase-separation member comprising a porous substrate of a low surfaceenergy material having void spaces therein, impregnated to at least halfof a thickness of said substrate with a microporous polymer material andcoated on an outer surface of said microporous polymer material with afluoropolymer layer applied so as to remain predominantly at said outersurface, said microporous polymer material forming a durable bondbetween said fluoropolymer layer and said low surface energy material,and said phase-separation member having a finer pore size than saidporous substrate alone.
 15. The phase separation member according toclaim 14, wherein said microporous polymer material is coagulatable andhas a viscosity in a range 300-1000 cP.
 16. The phase separation memberaccording to claim 14, further comprising hollow glass microbeads usedto fill the void spaces on said substrate.
 17. The phase-separationmember according to claim 14, wherein said porous substrate comprisesany one of a woven or nonwoven fabric, a knitted or a needle felt fibre,a porous film, a sintered sheet of synthetic particles, or a spiral linkfabric.
 18. The phase-separation member according to claim 14, whereinsaid microporous polymer material coagulates and impregnates said poroussubstrate substantially completely.
 19. The phase separation memberaccording to claim 14, wherein said fluoropolymer layer includes asynthetic fluorinated elastomer.
 20. The phase-separation memberaccording to claim 14, wherein said microporous polymer material is of anatural polymer which can be dissolved in a solvent.
 21. Thephase-separation member according to claim 14, wherein said microporouspolymer material is any one of a polyurethane, a silicone, afluoroelastomer, or a rubber.
 22. The phase-separation member accordingto claim 14, wherein said low surface energy material is a polyolefin.